As the porous thin paper used for a heat-sensitive stencil printing paper, there are generally known (1) a Japanese paper formed from a natural fiber such as paper mulberry, paper bush or manila hemp (Japanese Examined Patent Publication No. 41-7623), (2) a paper formed from a regenerated cellulose fiber or synthetic fiber such as rayon, vinylon, polyester or nylon, (3) a mixed paper from a mixture of the above-mentioned natural fiber and the above-mentioned regenerated cellulose fiber or synthetic fiber (Japanese Examined Patent Publication No. 49-18728), and (4) a polyester paper obtained by hot-pressing by a hot rot roll a thin paper formed from a mixture of a polyester fiber and an undrawn polyester fiber as a binder fiber (Japanese Examined Patent Publication No. 49-8809).
These thin papers are defective in that they are deformed by changes in the humidity and temperature to cause a change of the dimension or reduce the capabilities thereof. Accordingly, a method in which the change of the dimension in the wet state is reduced (Japanese Unexamined Patent Publication No. 61-254396) and a method in which a thin paper is impregnated with a liquid of a synthetic resin and is treated with the synthetic resin also acting as an adhesive for the thin paper and a film (Japanese Examined Patent Publication No. 55-47997) have been proposed. Furthermore, a method of forming a polyester paper having an excellent dimensional stability and heat resistance has been proposed (Japanese Unexamined Patent Publication No. 58-76597 and Japanese Unexamined Patent Publication No. 58-76598).
The thin paper to be used for a heat-sensitive stencil printing paper must have certain capabilities: i.e., (a) the ink permeability is good and the formed image is sharp, and the image capability is excellent, (b) the printing durability is excellent, (c) the paper strength is excellent and a falling of filaments is controlled, (d) little deformation such as thermal shrinkage or a formation of wrinkles occurs and printing can be an exact reproduction of an original.
None of the foregoing known thin papers, however, satisfies all of these requirements.
The problems involved in the conventional thin papers are summarized below.
The thin paper (1) using a natural fiber is unsatisfactory in that, although a dispersant or a tackifier is added at the paper-making step, "Japanese paper crush marks" based on an uneven dispersion of the fiber inhibit a permeation of an ink and defects or omissions appear in the formed image. Although a paper strength-increasing agent is generally added at the paper-making step, the paper strength is too low, and thus the base paper is wrinkled and the printing durability is poor.
In the case of the paper (2) formed from a regenerated cellulose fiber or the paper (3) formed from a mixture of a synthetic fiber and a natural fiber, the dispersion uniformity of the fiber is improved, but since the fixation of crossing points of the fiber is poor and the paper strength is low, deformation readily occurs and a falling of filaments is caused at the laminating or printing step, with the result that the image and the printing durability are poor. According to the method disclosed in Japanese Unexamined Patent Publication No. 61-254396, the dimensional stability in the wet state is improved by incorporating a polyester fiber or a regenerated cellulose staple fiber and adding an epoxidized polyamide-polyamine resin at the paper-making step. Nevertheless, this method is still unsatisfactory in that the fixation of crossing points of the fiber is poor and the printing durability and image are not satisfactory.
In the case of the thin paper (4) comprising a polyester paper, although the preparation process is contrived so that polyester filaments are tightly bonded to one another, many crossing portions of filaments are not bonded by the binder fiber and the fixation is poor. Furthermore, the thermal shrinkage caused by heat from a thermal head or the like is large, and because of a deformation or wrinkling, the printing of an exact reproduction of the original is impossible. Moreover, the bonding between the heat-sensitive film and thin paper at the laminating step is poor, and a problem arises of a partial peeling of the film, and as a result, the image and printing durability are poor.
According to the method of the synthetic resin processing disclosed in Japanese Examined Patent Publication No. 55-47997, a resin having a relatively low softening point is used for exerting the function of the adhesive between the thin paper and the film, and therefore, it cannot be considered that crossing points of the filaments are bonded with a strong resin having a heat resistance. Namely, since a softening of the resin occurs and the reinforcing effect is poor, the paper strength and printing durability are not satisfactory.